libstdc++: Fix -Warray-bounds warning in std::vector::resize [PR114945]

This is yet another false positive warning fix. This time the compiler
can't prove that when the vector has sufficient excess capacity to
append new elements, the pointer to the existing storage is not null.

libstdc++-v3/ChangeLog:

	PR libstdc++/114945
	* include/bits/vector.tcc (vector::_M_default_append): Add
	unreachable condition so the compiler knows that _M_finish is
	not null.
	* testsuite/23_containers/vector/capacity/114945.cc: New test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

2 files changed, 39 insertions, 0 deletions

diff --git a/libstdc++-v3/include/bits/vector.tcc b/libstdc++-v3/include/bits/vector.tcc
index 7a92f34..66d73b4 100644
--- a/libstdc++-v3/include/bits/vector.tcc
+++ b/libstdc++-v3/include/bits/vector.tcc
@@ -768,6 +768,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 
 	  if (__navail >= __n)
 	    {
+	      if (!this->_M_impl._M_finish)
+		__builtin_unreachable();
+
 	      _GLIBCXX_ASAN_ANNOTATE_GROW(__n);
 	      this->_M_impl._M_finish =
 		std::__uninitialized_default_n_a(this->_M_impl._M_finish,
diff --git a/libstdc++-v3/testsuite/23_containers/vector/capacity/114945.cc b/libstdc++-v3/testsuite/23_containers/vector/capacity/114945.cc
new file mode 100644
index 0000000..daafc59
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/vector/capacity/114945.cc
@@ -0,0 +1,36 @@
+// { dg-options "-O2 -Werror=stringop-overflow -Werror=array-bounds" }
+// { dg-do compile { target c++11 } }
+
+// Bug libstdc++/114945
+// Sporadic std::vector::resize() -Wstringop-overflow or -Warray-bounds warning
+
+#include <stdint.h>
+#include <vector>
+template <typename a> struct b {
+  void resize(std::size_t c) { d.resize(c); }
+  template <typename e> void f(a, e);
+  std::vector<char> d;
+};
+#include <regex>
+std::regex g;
+uint64_t h;
+uint32_t i;
+struct s {
+  enum class j : size_t;
+  void k();
+  using l = b<j>;
+  std::vector<l> m;
+};
+enum class s::j : size_t { n };
+void o() { g = ""; }
+void s::k() {
+  l p;
+  auto q = uint32_t(), r = uint32_t();
+  if (h)
+    r = i;
+  b<size_t> t;
+  if (q || r)
+    p.f(j::n, 5);
+  t.resize(4);
+  m.push_back(p);
+}